深井地电观测技术在地震监测中的应用探讨

Discussion on the deep well geoelectric observation technique applied in earthquake monitoring

天水地震台井下地电观测与常规的地面观测相比,减少干扰、提高信噪比,在近三年的地电阻率观测中,观测精度无超过0.5%数据,数据的内在质量较好.2013年4月10日开始,NS、EW道井下(100 m)观测的地电阻率出现了波动变化,持续11天后距该台550 km的四川芦山发生了M_S7.0级地震,5月4号芦山地震余震相对减少后,地电阻率资料的波动变化也同步终止;2013年6月11日18时开始,天水台地电阻率再次出现异常波动变化,7月22日距该台156 km发生岷县-漳县M_S6.6级地震.采用天水地电台井下不同时段(凌晨2点和全天)和不同深度(6 m和100 m电极)观测的地电阻率数据对比分析这两次地震前后地下介质的电性变化,深部记录到明显的异常变化;用小波(db4)分解发现在芦山M_S7.0和岷漳M_S6.6级地震前视电阻率的细节部分的形态是连续的振荡,尺度部分出现了趋势性似"漏斗"变化形态,与过去强震震例形态具有相似性、重复性.深井地电观测技术的应用,减小了地面离散电流影响,直接观测探测层介质的电性变化,是探索预报地震的新思路、新方法.

Compared with conventional surface observation, the geoelectric observation in deep well has a good data quality with less interference and higher SNR at Tianshui seismostation. The data accuracy is no more than 0.5 % during nearly three years electrical resistivity observation, and the result is smooth and steady. But the earth resistivity of both NS channel and EW channel fluctuated synchronously beginning in April 10, 2013, ten days later after obvious data change the Ms 7.0 earthquake occurred at Sichuan Lushan 550 km from Tianshui. After May 4th the Lushan earthquake is relatively reduced, and the variation of earth resistivity data is synchronized terminated; From eighteen in June II, 2013, the ground resistivity at Tianshui seismostation fluctuated abnormally again, and in June 20,2013 and in July 21 st, the heavy rainfall occurred at Tianshui twice, and Ms 6. 6 earthquake occurred at border of Minxian county and Zhangxian county 156 km from Tianshui in succession in July 22nd. These events such as earth resistivity change, earthquake events, the underground water level fluctuation, the extreme weather and other factors are complex interweave together, what is the meaning of the these events？ By carefully examine the interference of station environment, no significant interference source was found, and the precipitation in Tianshui area during nearly 4 years was analyzed and the statistics of groundwater level change was conducted, we found that the impact of these factors on the deep resistivity is very small; Observation data by using different times （2 in the morning and all day） and different depth electrodes （6 m and 100 m） are contrast analysed before and after two earthquake, changes of deep electric structure of subsurface have been observed; Then wavelet （db4） analysis showed that the changes of apparent resistivity is appeared, During Lushan Ms 7 earthquake the anomaly variation amplitude of NS reached 0. 3%, and EW up to 0. 28%, and during Ms 6. 6 earthquake occurred at border of Minxian county and Zhangxian county the anomaly variation amplitude of NS reached 0.4% , EW up to 0.43% ; there existed a continuous vibration anomalies in high frequency （detail changes ）and tendency anomalies in low frequency sub band （scale changes ）before and after twice earthquakes, while in different stage, the curve of the electrical resistance Like a funnel shape, the anomalous change are similar and reproducible compared with last major earthquakes have occurred. With the application of deep well geoelectric observation technology is less impacted by ground stray currents, direct observe the electricity structure of underground medium and change over time in resistivity of ground, the study of deep well resistivity is a new idea and method for earthquake monitoring and prediction.